As electrophotography, a method of forming an electrostatic charge image on a photosensitive material comprising a photoconductive substance, developing the electrostatic charge image by a toner carried on a developing roller, transferring the toner image developed on the photosensitive material directly to a recording medium, e.g., paper, or via an intermediate transfer substance, and fixing the toner image on the recording medium by a fixing roller, e.g., a heating roller, on the recording medium, e.g., paper, by press-heating is known.
The toners used in this method are required not to bring about a so-called low temperature or hot offset, i.e., the adhesion of melted toner on a heating roller, and also required to have excellent fixing ability such as great fixing strength of the toner image fixed on a recording medium.
In fixing using a heating roller, as the factors which control the fixing ability and the offset resistance of the toners, it is well known that the storage modulus G′ and the loss modulus G″ in dynamic viscoelastic characteristics of a toner have influence. Storage modulus G′ and loss modulus G″ are viscoelastic characteristics of a substance having general viscoelasticity defined by complex elastic modulus in vibration experiment, and the real number part of complex elastic modulus is called storage modulus G′ and the imaginary number part is called loss modulus G″, specifically, storage modulus is an index showing the degree of the elasticity of a toner and loss modulus is an index showing the degree of viscosity. The dynamic viscoelastic characteristics are characteristics having a temperature-dependency varying according to the temperature, a frequency-dependency varying according to the frequency, and a strain-dependency varying according to the strain, i.e., characteristics showing a linear region of behaving linearly according to temperature, frequency and strain, or a nonlinear region of behaving nonlinearly.
It is proposed to improve the fixing ability, offset resistance and blocking resistance of a toner image by expressing the melting state of a toner at fixing time in such dynamic viscoelastic characteristics of temperature-dependency of a toner (e.g., refer to patent literature 1).
That is, the toner in this proposal is the toner containing binder resins, colorants and release agents, and the proposal, intends, to improve low temperature fixing ability, offset resistance and blocking resistance of the toner by setting the temperature of the time when the ratio of loss modulus to storage modulus (G″/G′=tan δ) becomes 1.0 at the range of from 55 to 70° C., the elastic modulus at that time at 1.5×108 Pa or less, the ratio of storage modulus G′ (40) to storage modulus G′ (50) (G′ (40)/G′ (50)) at from 1.5 to 5.0, the ratio of storage modulus G′ (50) to storage modulus G′ (60) (G′ (50)/G′ (60)) at from 3 to 20, the ratio of storage modulus G′ (70) to storage modulus G′ (100) (G′ (70)/G′ (100)) at from 50 to 250, and the ratio of storage modulus G′ (110) to storage modulus G′ (140)(G′ (110)/G′ (140)) at from 2 to 20.
[Patent literature 1]
JP-A-10-171156 (“Abstract” etc.) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
In the above-described fixation by heating, toners come to show the behavior of a linear region (L1) before fixing nip (inlet), the behavior of a nonlinear region (NL) S at fixing nip part, and the behavior of a linear region (L2) at the outlet of fixing nip.
However, in the toner disclosed in patent literature 1, the dynamic viscoelasticity of temperature-dependency measured in a linear region is used. In the fixation by heating, as described above, mere application of the dynamic viscoelasticity of temperature-dependency measured in a linear region to the toner showing a linear region (L1)—a nonlinear region (NL)—a linear region (L2) behavior is not conformable to actual behavior of the toner at the time of heat-fixation. Therefore, it cannot be said that low temperature fixing ability and offset resistance of the toner are sufficiently and effectively improved.
Thus, it cannot be said that sufficient and effective improvement has been done by conventional improvement of fixing characteristics of toners, and there is plenty of scope for improvements of surface smoothness of the fixing surface, fixing strength of a toner, prevention of fattening of characters, transparency, low temperature fixing ability of toners.
The present invention has been done in view of these circumstances.